In an image forming apparatus such as a copying machine, when a print key turns on, light is applied to a document set on a document glass and its reflected optical image is projected onto the light-receiving surface of a CCD (Charge Coupled Device) and converted into an electrical signal.
The light-receiving surface of the CCD has a number of pixels arranged in rows (horizontal direction) and columns (vertical direction). The CCD scans a document in the main and sub-scanning directions and outputs an image signal (analog signal) having a voltage level corresponding to the density of an optical image projected on the light-receiving surface.
An image forming apparatus capable of copying a color image includes a three-line sensor constituted by three line sensors (first to third line sensors) arranged at regular intervals (each corresponding to, e.g., eight lines) in the sub-scanning direction. A red-colored filter through which a red light beam passes is mounted on the first line sensor, a green-colored filter through which a green light beam passes is mounted on the second line sensor, and a blue-colored filter through which a blue light beam passes is mounted on the third line sensor. Firstly, the first line sensor outputs an image signal corresponding to a red-colored image. Secondly, the second line sensor outputs an image signal corresponding to a green-colored image. Lastly, the third line sensor outputs an image signal corresponding to a blue-colored image.
The image signal output from the first line sensor is converted to a digital signal serving as image data R by an A/D converter. The image signal output from the second line sensor is converted to a digital signal serving as image data G by an A/D converter. The image signal output from the third line sensor is converted to a digital signal serving as image data B by an A/D converter. Image processing is executed on the basis of the image data R, G and B to obtain a plurality of items of image data Y (yellow), M (magenta), C (cyan) and K (black) corresponding to a plurality of desired colors. These image data items Y, M, C and K drive a laser beam generator to start printing with a laser beam generated therefrom.
Since, as described above, the first to third line sensors are arranged at regular intervals, image data G is acquired after a lapse of a fixed time period after image data R is obtained. Similarly, image data B is acquired after a lapse of a fixed time period after image data G is obtained.
Consequently, in order to print a color image properly, the image data R and G are delayed and held in a memory until the image data B is obtained, and image processing is executed when the image data R, G and B are completed. Printing is thus started based on the image data items Y, M, C and K obtained from the image processing.
If, however, the above delay processing is performed when a scanning magnification is changed in the sub-scanning direction, there occurs a problem in which a writing position in the sub-scanning direction. (a printing start position in the sub-scanning direction of an image to be copied on paper) is shifted.
In other words, when the scanning magnification is 100%, a period of time required for delay processing due to a physical position of the line sensors corresponds to, for example, eight lines. However, when the magnification becomes 400%, the scanning speed is decreased to one quarter and accordingly the time period for delay processing corresponds to 32 lines.
In the three-line sensor, usually, the output timing of the sensors (the red-colored and green-colored line sensors) is adjusted in consistency with that of the sensor (the blue-colored line sensor) whose output is the last. When the scanning magnification is 100%, a delay process for 16 lines (=8×2) is required until the outputs of the three line sensors are completed, while a delay process for 64 lines (=8×4×2) is required when the magnification is 400%. The lines required to the delay process differ in number between the scanning magnification of 100% and that of 400% by 48 (=64−16).
In a prior art image forming apparatus, the timing at which a resist roller of a printer unit sends paper sheets to a photosensitive drum is set to that required when the scanning magnification is 100%. If, therefore, the magnification is changed to a value other than 100%, an image to be printed on the paper sheets deviates (from a position in the paper feeding direction or a writing position in the sub-scanning direction).
Assume, as a specific example, that the writing timing of a copied image is set so as to allow a margin (void width) of, e.g., 5 mm from the top of paper in the sub-scanning direction, taking into consideration that a delay process for 16 lines is executed when the scanning magnification is 100%. If the scanning magnification is changed to 400% under this assumption, the writing timing is delayed by 48 lines (=64−16), with the result that a margin (void width) of 5 mm is expanded by 48 lines (=8×4×2−16).
If the printing resolution is, for example, 600 dpi, the margin is increased by about 2 mm, and the increased margin becomes 7 mm (5 mm+2 mm) when the scanning magnification is 400% under the above assumption. In other words, if the scanning magnification changes from 100% to 400%, the margin (void width) deviates by 2 mm.